The present invention relates to a reflection-type liquid crystal display device, and in particular to a reflection-type liquid crystal display device employing a Guest Host (GH) type display mode using a reflecting polarizer.
Conventionally known is a reflection-type liquid crystal display device capable of modulating incident light between scattering (bright state) and absorption (dark state) by an application of a voltage, by dispersing a polymer having an anisotropic scattering function in a liquid crystal element employing a GH-type display mode in which a dichroic dye (Guest) having anisotropy in absorption of visible light in respective directions of a long axis and a short axis of a molecule is mixed with a liquid crystal (Host) having a certain molecular arrangement. The following will explain the conventional reflection-type liquid crystal display device as above based on FIG. 9.
A reflection-type liquid crystal display (LCD) device 101 shown in FIG. 9 is made up of a first substrate 102, a second substrate 103 and a liquid crystal layer 104 which is a complex layer composed of the dichroic dye, liquid crystal and polymer, sandwiched between the first substrate 102 and second substrate 103.
The first substrate 102 includes an insulating plate 105a which is a substrate, an absorbing later 106, a reflecting polarizer 107, a transparent electrode 108a and an orientation membrane 109a, which are provided in this order from the side of the insulating plate 105a and between the insulating plate 105a and the liquid crystal layer 104. Note that, in FIG. 9, 110 is a seal material.
The second substrate 103 includes an insulating plate 105b as a substrate, a transparent electrode 108b, an orientation membrane 109b, which are provided in this order from the side of the insulating plate 105b and between the insulating plate 105b and the liquid crystal layer 104.
The liquid crystal layer 104 which is placed between the first substrate 102 and second substrate 103 is made up of a liquid crystal molecule 104a of a nematic liquid crystal having a positive dielectric anisotropy, a so-called p-type dichroic dye 104b having a transitional dipole moment which is substantially parallel to the long axis of the molecule, and a polymer of polymerized molecules (polymer) 104c having an anisotropic molecular skeleton.
The insulating plate 105a of the first substrate 102, and the insulating plate 105b of the second substrate 103 are composed of an insulating material, for example, such as glass, quartz and plastic. Further, at least the insulating plate 105b of the second substrate 103 is formed of a material having light transmissivity.
On a surface of the insulating plate 105a of the first substrate 102 on the side of the liquid crystal layer 104 are formed the absorbing layer 106 in contact with the insulating plate 105a as explained, and the reflecting polarizer 107 which is made up of, for example, a dielectric multilayer membrane having birefringence. Here, when forming the reflecting polarizer 107, a transmitted axis of the liquid crystal layer 104 and that of the reflecting polarizer 107 are lined up with each other.
Further, in the first substrate 102, the orientation membrane 109a which is provided on a surface of the reflecting polarizer 107 via the transparent electrode 108a in between, and the orientation membrane 109b which is provided under a surface of the insulating plate 105b via the transparent electrode 108b in between are made of, for example, polyimide resin. Furthermore, on respective surfaces of these orientation membranes 109a and 109b, on the sides which are in contact with the liquid crystal layer 104, an orientation treatment, for example, by rubbing is performed so as to orient the liquid crystal molecule 104a of the nematic liquid crystal horizontally in one direction with respect to the first substrate 102 and second substrate 103.
Next, the following will explain an operation when performing black and white display by using the reflection-type LCD device 101 with reference to FIGS. 10(a) and 10(b). FIG. 10(a) shows a state of the reflection-type LCD device 101 when applying no voltage while FIG. 10(b) shows a state of the reflection-type LCD device 101 when applying a voltage. Note that, light 111 which is emitted from surroundings (surrounding light) is indicated by linearly polarized light 111a having one polarization direction (oscillation direction) and linearly polarized light 111b having another oscillation direction which orthogonally intersects the former oscillation direction.
As shown in FIG. 10(a), when applying no voltage, the liquid crystal molecule 104a of the liquid crystal layer 104 is oriented along an orientation direction of the orientation membranes 109a and 109b, that is, in a direction parallel to the first substrate 102 and second substrate 103. In addition, the p-type dichroic dye 104b of the liquid crystal layer 104 is oriented in the same manner as the liquid crystal molecule 104a. 
When the light 111 which is incident from the side of the second substrate 103 is incident on the liquid crystal layer 104, a component of the light 111, i.e. the linearly polarized light 111a having its oscillation direction in a direction parallel to the long axis direction of the molecule of the p-type dichroic dye 104b is absorbed by the p-type dichroic dye 104b. Some of the linearly polarized light 111a cannot be absorbed by the p-type dichroic dye 104b and is transmitted. However, since the linearly polarized light 111a thus being transmitted through the liquid crystal layer 104 was scattered by the polymer 104c and became scattering light, it is reflected at the reflecting polarizer 107 and absorbed by the p-type dichroic dye 104b when passing through the liquid crystal layer 104 again.
Further, the linearly polarized light 111b having an oscillation plane in a vertical direction with respect to the long axis direction of the molecule of the p-type dichroic dye 104b passes through the liquid crystal layer 104 and reflecting polarizer 107, and is absorbed by the absorbing layer 106 behind the reflecting polarizer 107.
Thus, most of the linearly polarized light 111a do not emerge but absorbed by the liquid crystal layer 104 and absorbing layer 106. Accordingly, when applying no voltage, most of the light which is incident on the reflection-type LCD device 101 is absorbed by the reflection-type LCD device 101, thereby resulting in a dark state.
On the other hand, as shown in FIG. 10(b), when applying the voltage, the nematic liquid crystal molecule 104a and p-type dichroic dye 104b of the liquid crystal layer 104 rise along a direction of the voltage, and are oriented in the vertical direction with respect to the first substrate 102 and second substrate 103. However, the polymers 104c are chemically bound to one another so that a direction thereof cannot be changed. As a result, there arises a difference in refractive index between an area composed of the liquid crystal molecule 104a and p-type dichroic dye 104b, the molecules of which rose along the direction of the voltage, and an area composed of the polymers 104c, molecules of which did not rise. For this reason, the light incident on the liquid crystal layer 104 takes the scattering state.
Namely, when the light 111 incident from the side of the second substrate 103 is incident on the liquid crystal layer 104, a component of the light 111, i.e. the linearly polarized light 111b having its oscillation direction in the vertical direction with respect to the long axis direction of the molecule of the p-type dichroic dye 104b passes through the liquid crystal layer 104 and reflecting polarizer 107, and is absorbed by the absorbing layer 106 behind the reflecting polarizer 107.
In addition, some of the linearly polarized light 111a having its oscillation direction in a direction parallel to the long axis direction of the molecule of the p-type dichroic dye 104b is scattered at the polymer 104c, while the other is reflected at the reflecting polarizer 107, and thereafter, passes through the liquid crystal layer 104 again to emerge, thereby showing a bright state.
A reflecting polarizer which is used as the reflecting polarizer 107 is a dielectric multilayer film having birefringence, which has a characteristic to reflect linearly polarized light having its oscillation plane in a direction of travel while transmitting linearly polarized light having its oscillation plane in a direction which orthogonally intersects the direction of travel. This reflecting polarizer is disclosed in Published Japanese Translation of PCT International Publication No. WO95/17303 for Patent Application No. PCT/US94/14323 (Tokuhyohei 9-506837 published on Jul. 8, 1997).
Incidentally, a device employing the reflecting polarizer as above is disclosed in a catalog OPP-049-A (049803)TY published by Sumitomo 3M Ltd. (3M) or in Electronic Display Forum 98 (pages 4-16).
In addition, the conventional reflection-type LCD device as discussed is disclosed, for example, in Japanese Unexamined Patent Publication No. 38452/1999 (Tokukaihei 11-38452 published on Feb. 12, 1999).
However, in such a structure as to provide the reflecting polarizer and absorbing layer in the order shown in the foregoing conventional reflection-type LCD device, even in the bright state, the linearly polarized light which oscillates in the orthogonal direction to the orientation direction of the liquid crystal molecule passes through the liquid crystal layer, which is a dichroic-dye/liquid-crystal/polymer complex layer, and the reflecting polarizer, and is absorbed by the absorbing layer provided behind the reflecting polarizer. Consequently, even in the bright state, about a half of the incident light is absorbed by the absorbing layer, thereby raising a problem that bright display cannot be attained in effect.
In view of the foregoing problem, it is an object of the present invention to provide a reflection-type liquid crystal display device capable of bright and high-contrast display.
In order to attain the foregoing object, a reflection-type LCD device of the present invention is made up of:
a first liquid crystal display layer having a first liquid crystal layer which includes a liquid crystal material, a dichroic dye and an anisotropic scattering material;
a reflecting polarizing layer which is disposed to transmit linearly polarized light having a polarization direction in a transmitted axis direction of the first liquid crystal display layer; and
a second liquid crystal display layer including a polarizer which is disposed to transmit the linearly polarized light transmitted through the reflecting polarizing layer, a second liquid crystal layer capable of bright and dark display depending on presence or absence of an applied voltage, and a reflecting layer,
wherein the first liquid crystal display layer, reflecting polarizing layer and second liquid crystal display layer are stacked in this order from a side from which light is incident.
When the dichroic dye (Guest) is mixed with the liquid crystal material (Host), a long narrow molecule of the dichroic dye is aligned parallel to the molecule of the liquid crystal material. Accordingly, when the molecular alignment of the liquid crystal molecule is changed by applying an electric field, the molecular alignment of the dichroic dye is also changed with it, thus automatically controlling the absorbed quantity of visible light by the dichroic dye. A display mode of the liquid crystal display element thus utilizing an electro-optical effect is called a Guest Host (GH) type display mode. Combining the anisotropic scattering material which scatters linearly polarized light having a polarization direction (oscillation direction) in the same direction as an orientation direction of the liquid crystal material and dichroic dye with the first liquid crystal layer employing the GH-type display mode as above enables the first liquid crystal display layer to modulate scattering and transmission of the incident linearly polarized light by an application of the electric field.
The reflecting polarizing layer which is stacked on the first liquid crystal display layer is disposed to transmit linearly polarized light of one direction which is transmitted through the first liquid crystal display layer including the first liquid crystal layer as shown above, i.e. the linear polarized light which has one polarization direction (oscillation direction) coinciding with the transmitted axis direction of the first liquid crystal display layer. Namely, the reflecting polarizing layer is disposed in such a manner as to transmit linearly polarized light having one polarization direction as above, and reflect the other linearly polarized light having a polarization direction orthogonally intersecting it. Furthermore, the polarizer making up the second liquid crystal display layer is also disposed so as to transmit the linearly polarized light of one direction which passes through the reflecting polarizing layer. Further, since the liquid crystal layer capable of bright and dark display depending on presence or absence of the applied voltage is used as the second liquid crystal layer, the linearly polarized light which passes through the polarizer to be incident on the second liquid crystal layer either becomes circularly polarized light or passes through while maintaining its polarization state, depending on presence or absence of an applied voltage.
Suppose that the arrangement of the reflection-type LCD device as explained is a first arrangement of the present invention.
Here, the following will explain an operation of the reflection-type LCD device according to the present invention in the case where the liquid crystal material and dichroic dye are, for example, aligned in the orientation direction (here, suppose that it is parallel to a substrate) when applying no electric field, while they are aligned in a direction orthogonally intersecting the orientation direction (here, suppose that it intersects perpendicularly to the substrate) when applying the electric field.
First, the following will explain the case where the voltage is not applied to both of the first and second liquid crystal layers.
When applying no voltage, the linearly polarized light incident on the first liquid crystal layer, i.e. linearly polarized light having a polarization direction (oscillation direction) in the same direction as the orientation direction of the liquid crystal material and dichroic dye, is either absorbed by the dichroic dye when passing through the first liquid crystal layer or scattered by the anisotropic scattering material. The scattering light which passed through the first liquid crystal layer is thereafter reflected at the reflecting polarizing layer, and then absorbed by the dichroic dye when passing through the first liquid crystal layer again, and thus essentially no light emerges from the device.
On the other hand, the linearly polarized light having a polarization direction orthogonally intersecting that of the above linearly polarized light is transmitted through the first liquid crystal layer without being absorbed by the dichroic dye even upon incidence on the first liquid crystal layer. Thereafter, the linearly polarized light is also transmitted through the reflecting polarizing layer and the polarizer which makes up the second liquid crystal display layer so as to enter the second liquid crystal layer. When the second liquid crystal layer is, for example, made of a twisted nematic liquid crystal which changes the linearly polarized light into the circularly polarized light when applying no voltage, this circularly polarized light becomes circularly polarized light rotating in the reverse direction, after being reflected at the reflecting layer. Consequently, when the circularly polarized light rotating in the reverse direction is incident on the polarizer again, after passing through the second liquid crystal layer, it becomes linearly polarized light having the polarization direction which is different by 90xc2x0 with respect to the transmitted axis of the polarizer, and thereby the light is blocked by the polarizer.
Thus, in the state where the voltage is not applied, linearly polarized light having any polarization directions is absorbed and it does not emerge from the device, thereby realizing the dark state.
Next, the following will explain the case where the voltage is applied to both the first and second liquid crystal layers.
When applying the voltage, since the molecules of the liquid crystal material and dichroic dye both rise in the direction to intersect perpendicularly to the substrate, the foregoing linearly polarized light (linearly polarized light having the polarization direction in the same direction as the orientation direction of the liquid crystal material and dichroic dye) is not absorbed but scattered by the anisotropic scattering material when passing through the first liquid crystal layer, then, reflected at the reflecting polarizing layer and scattered by the first liquid crystal layer again, so as to emerge from the device.
On the other hand, the linearly polarized light whose polarization direction orthogonally intersects that of the above linearly polarized light is transmitted through the first liquid crystal layer, reflecting polarizing layer and polarizer, then, incident on the second liquid crystal layer. Since a phase difference does not occur in the second liquid crystal layer when applying the voltage, the linearly polarized light is transmitted through the second liquid crystal layer while maintaining its polarization state, and reflected at the reflecting layer, and thereafter, it is transmitted again through the polarizer, reflecting polarizing layer and first liquid crystal layer, so as to emerge from the device.
As discussed, when applying the voltage, linearly polarized light having any polarization directions can emerge from the device, thereby contributing to a bright state of display.
Consequently, the dark state of display can surely be realized, while making sure, in the bright state, that the linearly polarized light of one direction, which was absorbed in the conventional arrangement, can emerge from the device, i.e. both rays of the linearly polarized light incident on the reflection-type LCD device can emerge from the device.
Note that, in both of the first and second liquid crystal layers, a positive liquid crystal material which allows the molecules of the liquid crystal material and dichroic dye to rise with respect to the voltage has been used through the foregoing explanation, but a negative liquid crystal material can undoubtedly be adopted as well.
Thus, a reflection-type LCD device having desirable display quality with highly improved brightness and contrast can be realized.
Further, in order to solve the foregoing problems, the reflection-type LCD device of the present invention may have an arrangement which includes:
a first liquid crystal display layer having a first liquid crystal layer including a liquid crystal material, a dichroic dye and an anisotropic scattering material;
a reflecting polarizing layer which is disposed to transmit linearly polarized light having a polarization direction in a transmitted axis direction of the first liquid crystal display layer; and
a second liquid crystal display layer including a polarizer which is disposed to transmit the linearly polarized light transmitted through the reflecting polarizing layer, a second liquid crystal layer capable of bright and dark display depending on presence or absence of an applied voltage, and a reflecting layer,
wherein the first liquid crystal display layer, the reflecting polarizing layer and the second liquid crystal display layer are stacked in this order from a side from which light is incident.
Referring to this arrangement as a second arrangement of the reflection-type LCD device according to the present invention, the second arrangement realizes the first liquid crystal display layer by providing an anisotropic scattering membrane separately from the first liquid crystal layer and in replacement of the anisotropic scattering material in the first liquid crystal layer in the reflection-type LCD device having the first arrangement of the present invention, where the anisotropic scattering membrane is for transmitting linearly polarized light having a polarization direction (oscillation direction) in the same direction as an orientation direction of molecules of the liquid crystal material and dichroic dye of the first liquid crystal layer, and scattering linearly polarized light having a polarization direction which orthogonally intersects that of the above linearly polarized light. Accordingly, the reflection-type LCD device having the second arrangement of the present invention can also obtain the same effects as those of the reflection-type LCD device having the first arrangement. That is, while surely realizing the dark state of display, in the bright state, the linearly polarized light which was absorbed in the conventional arrangement emerges from the device, thus resulting in emergence of both rays of the linearly polarized light incident on the reflection-type LCD device.
Thus, the reflection-type LCD device having desirable display quality with highly improved brightness and contrast can be realized.
Additional objects, features, and strengths of the present invention will be made clear by the description below. Further, the advantages of the present invention will be evident from the following explanation in reference to the drawings.